fwu: mtd: get MTD partition specific info from driver

Information about FWU images on MTD partitions is now stored with the
corresponding driver instead of a global variable. Get this
information from the driver.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>